1. Field of the Invention
This invention relates to a photo-sensor for reading information such as characters and patterns. The photo-sensor of this invention is useful as the linear image sensor of a facsimile equipment or optical character recognition, etc.
2. Description of the Prior Art
Heretofore, a linear silicon photodiode array has been generally employed as the photosensitive element of a facsimile transmitter, optical character recognition etc. Since, however, silicon is subject to limitations in the size of a producible single crystal and in the processing technique, the length of the linear silicon photodiode array has a limit. At present, the length of the linear silicon photodiode array achievable is only about 30 mm at the utmost. On the other hand, an original picture to be read has a width of, for example, 210 mm in the A4-size. Accordingly, in case of reading the original picture of the A4-size with the linear silicon photodiode array, the original picture is scaled down and imaged on the linear silicon photodiode array by the use of a lens system.
FIG. 1 is a view for explaining this principle. Numeral 1 designates an original picture, numeral 2 a lens, and numeral 3 a linear silicon photodiode array. In this case, a certain distance is inevitably required between the original picture and the linear silicon photodiode array. This is very unfavorable for the miniaturization of the input device. In addition, in case of using such a lens system, there are the disadvantages that the positioning of the lens requires much labor and that a degraded resolution of a peripheral part and an insufficient quantity of light arise.
Recently, it has been attempted to eliminate the disadvantages of such a lens system by employing optical fibers as an optical system.
An example of the attempt is described in detail in, for example, "Gazo Denshi Gakkai Shi (Bulletin of the Picture Electronics Society)," vol. 4, No. 2, pp. 54-61 (1975). Hereunder, the technique will be briefly explained.
FIG. 2 is a view showing the operating principle of the prior art. In the figure, numeral 4 designates an original picture, and numeral 5 denotes 1,280 optical fibers each having a diameter of 125 .mu.m. One side of the optical fibers close to the original picture is in the form of a sheet, while the other side is distributed to 20 linear silicon photodiode arrays (S.sub.1 -S.sub.20). The linear silicon photodiode array consists of 64 photodiodes. FIG. 3 is an enlarged sectional view of the joined part between the optical fibers and the linear silicon photodiode array. In this figure, numerals 61-66 indicate optical fibers, and symbols S.sub.1 -S.sub.6 silicon photodiodes. The pitch of the silicon photodiodes is indicated by D, the spacing between the silicon photodiode and the optical fiber is indicated by d.sub.1, and the misalignment between the silicon photodiode and the optical fiber is indicated by d.sub.2. The optical fibers and the photodiodes correspond at 1:1. In order to prevent the breakage of the photodiode, a clearance d of about several hundreds .mu.m needs to be provided between the optical fiber and the photodiode. The clearance lowers the resolution of the photo-sensor drastically. After all, the photo-sensor having such a structure is meritorious over the photo-sensor employing the lens system in that the degradation of the resolution of the peripheral part and the insufficient quantity of light can be avoided, but it is not practical on account of disadvantages as listed below.
1. The optical fibers of the structure, in which one side is in the form of the very thin sheet and the other side is distributed to the 20 linear silicon photodiode arrays, eventually needs some extent of length. This is unfavorable for the miniaturization of the device. Moreover, there is a high possibility that the optical fibers will be broken by mechanical shocks such as vibrations.
2. Inasmuch as the optical fibers must be arrayed in perfect conformity with the pitch of the silicon photodiodes, fibers of high precision are required. The alignment takes much labor, and the position is prone to shift in the course of use.
3. In order to prevent the destruction of the linear silicon photodiode array, the optical fibers must be floated in use. This degrades the resolution drastically.
There will be explained how the spacing d.sub.1 between the silicon photodiode and the optical fiber and the misalignment d.sub.2 between them ought to be controlled in case of the device of this system.
When the spacing d.sub.1 is varied, the ratio between the outputs of the silicon photodiodes S.sub.4 and S.sub.5 as based on light from the optical fiber 65 varies, by way of example, as follows: EQU At d.sub.1 =20 .mu.m, S.sub.5 /S.sub.4 .about.14. EQU At d.sub.1 =50 .mu.m, S.sub.5 /S.sub.4 .about.10. EQU At d.sub.1 =100 .mu.m, S.sub.5 /S.sub.4 .about.5.
(In these examples, D=125 .mu.m and d.sub.2 =0 .mu.m)
When the misalignment d.sub.2 is varied, the ratio between the outputs of the silicon photodiodes S.sub.2 and S.sub.1 as based on light from the optical fiber 62 varies, by way of example, as follows: EQU At d.sub.2 =5 .mu.m, S.sub.2 /S.sub.1 .about.10. EQU At d.sub.2 =25 .mu.m, S.sub.2 /S.sub.1 .about.4. EQU At d.sub.2 =45 .mu.m, S.sub.2 /S.sub.1 .about.2.
(In these examples, D=125 .mu.m and d.sub.1 =50 .mu.m)
In order to separate the information of the adjacent optical fibers, therefore, an adjustment of considerably high precision is required. Since the picture information is turned into a binary signal, it is required in practice to make d.sub.2 within .+-.10 .mu.m at d.sub.1 =50 .mu.m. The dimensional precision for this control is considerably high. In addition, the requirement for the dimensional precision becomes still higher to the end of enhancing the resolution, and a resolution of 4-5 lines/mm will be a limitation in actuality.